Study On Preparation Of Open Framework Fluoride Electrode Materials And Their Application In Na-ion Batteries

Room temperature Na-ion batteries（NIBs） have large potential application in large-scale stationary energy storage owing to potentially better rate performance apart from resource and they are receiving increasing attentions. Currently, the preparation of electrode materials for NIBs remarkably depended on the modification or expansion of existing structure prototypes. Open framework strategy has been used to increase size or dimension of Na-ion channels, to alleviate the generation of staged phases and multiphase interfaces during cycling and to increase the diffusion rate of Na-ion.（1） Here we report a novel bronze phase（tetragonal tungsten bronze, TTB） of oxyfluoride（KNb₂O₅F） as a potential NIB anode for the first time. We obtain this tunnel-type open framework through equimolar KF doping into T-Nb₂O₅ with K and F as channel supporter and ligand substituent respectively. The intrinsic conductivity of the sample is improved and it has a characteristic of near-zero strain sodiation. These contribute to a highly reversible capacity and good rate performance even without nano- or defect-engineering. This open framework also brings about a substantial capacitive contribution which is called intercalation pseudocapacitance.（2） Here we also report a novel bronze phase（tetragonal tungsten bronze, TTB） of iron-based fluoride as potential cathode for NIBs. This tunnel-type fluoride is prepared by topotactically densifying a hexagonal tungsten bronze（HTB） framework with intrinsic corner-sharing Fe F6 octahedral chains. K-stuffed TTB fluoride realizes a favorable electrochemical K extraction with a first charge capacity as high as 125 mAh/g and the following reversible Na-storage with discharge capacity of 100¹50 mAh/g. Structure integrity and near-zero strain sodiation are also responsible for the good cyclability.